Surly Flask Funk

I've had a Surly flask for almost a year- put some stranny's in it last spring, used it a few times, then it sat for probably a month before I cleaned it out. Problem is, ever since, it has some black isht in it and little chunks of it come out if I pour liquid in (found this out because I just went to refill it with more stranny's and when I took a swig, there was something in it ).

What care do y'all who have flasks take of your flasks? Do you clean them regularly?

1
Pour one cup of water into the liquid measuring cup. Add one cup of the distilled white vinegar.

2
Mix the water and vinegar together in the measuring cup, using the spoon. Mix until the two liquids are thoroughly combined.

3
Boil water and fill the hip flask with the boiling water. Pour it out, and repeat one more time. Be careful not to burn yourself in this process, and hold the metal flask with a towel or pot holder if necessary to protect yourself.

4
Fill the flask about halfway with the water and vinegar solution. Close the top and shake the flask vigorously, for about 20 seconds.

5
Pour out the solution. Repeat Step 3 and Step 4, rinsing alternatively with boiling water, then the vinegar solution. After repeating these steps, Fill the flask with boiling water one more time, to clear out any residual vinegar in the hip flask. Turn upside down on a paper towel to dry. The water will come out and be absorbed by the paper towel.